My B12 was 118 pg/mL. I received Casinerve Plus (methylcobalamin 1500 mcg IM) in my buttock on Friday evening. From Saturday afternoon, I developed intermittent shortness of breath/air hunger. Last night it became quite severe with chest heaviness. Sometimes when I take repeated deep breaths, I feel dizzy. My SpO₂ and pulse have been normal, and BP has generally been normal, sometimes slightly low. I want to know whether this could be a reaction to the B12 injection or another cause, and whether I should avoid another injection and consider oral B12 or another formulation.